1. Field of Invention
This invention relates to the generation of low-cost, high-efficiency, high-power laser light for many applications using semiconductor diode lasers combined with their driver circuitry, both of which are immersed in a cryogenic cooling fluid such as liquid nitrogen (77 K) or others.
2. Discussion of Prior Art
A problem with conventional magnetic resonance proton imaging (MRI) is that airspaces such as in the lung, throat, etc. cannot be imaged [1-5]. Therefore, human pulmonary airway diseases (cystic fibrosis, etc.) could not be diagnosed by MRI in the past. But this situation is changing. The status as explained by the experts in the field [3] is as follows: xe2x80x9cThe ability to optically polarize the nuclei of the inert gases helium-3 and xenon-129 to 30% to 50% levels has made available a powerful new signal source for magnetic resonance imaging examinations. The non-equilibrium nuclear polarization of this hyperpolarized (HP) gas can be as much as 104-105 times larger than the equilibrium polarization of the hydrogen protons in water used in conventional MR imaging experiments. Even though the spin density of the gases (at 1 atm) is roughly 3,000 times smaller than that of water, this still represents a substantial net gain in signal amplitudexe2x80x9d [3]. Hyperpolarized (HP) gas is generated by light xe2x80x98pumpingxe2x80x99 with semiconductor diode lasers. xe2x80x9cNot surprisingly, the amount of gas that can be polarized is ultimately limited by the laser power that is available.xe2x80x9d [3]. The light output power of the pumping diode lasers for the generation of the hyperpolarized gases is in the order of magnitude of 10-150 watt [6]. Diode lasers are cost effective by an order of magnitude compared to Ti:sapphire lasers. But at these output levels they are nevertheless still very expensive. A 500 mW laser diode may cost as much as $200-$500 depending on quantities. Important new laser diode applications as discussed here may require 100 W at a cost of $40,000 to $80,000. Therefore, an attempt must be made to reduce their cost in order to permit a widespread introduction of this new promising hyperpolarized gas magnetic resonance imaging technique into many hospitals. Thus, one can state that prior art is expensive, inefficient, and of low power output.
In addition to the use of diode lasers for inert gas polarization and generation there are many other applications for these sources of laser light to be discussed later.
Measurements on light-emitting diodes (LEDs) show that their light output if immersed in liquid nitrogen (LN2, 77 K) can be one to two orders of magnitude higher than at room temperature (300 K) for the same diode current. In FIGS. 1 and 2 the output for a yellow LED as a function of the diode current is plotted for 300 K and 77 K, demonstrating the increase in efficiency by cryo-cooling. From these measurements, one can conclude that GaAs laser diodes may behave in a similar fashion. This is proven for a red diode laser in FIG. 3. If by cryo-cooling the output power of a laser diode can be increased by a factor N, then the cost for a given power level is reduced by a factor N, neglecting, in a first order approximation, the cooling penalty cost.
Very high current and power densities occur in diode lasers. Improved thermal management is therefore extremely important. FIGS. 4 and 5 demonstrate how cryo-cooling drastically increases the thermal conductivity of semiconductor materials and usual substrates such as beryllium and beryllium-oxide. Therefore cryo-cooled diodes will have smaller temperature gradients, which improves the lasing action as well as the thermal management of the system. Note that according to FIG. 5, the thermal conductivity of beryllium and BeO at 77 K is higher than that of copper.
A key object of this invention is to integrate the driver circuitry with the diode laser in a cryogenic environment, such as a bath of liquid nitrogen (LN2) at 77 K. In addition to the improved performance of the semiconductor laser, one obtains all the advantages of cryo-cooled power electronics based on the use of power Cryo-MOSFETs and Cryo-COOL-MOS devices: Higher efficiency, higher speed, longer lifetime, higher reliability, smaller size, etc. [27-31]. It has been found that certain integrated CMOS circuits such as the TC4422/21 (9 A) drivers are well suited for cryogenic operation. Cryo-cooling increases their efficiency by reducing their current consumption. In addition, their switching speeds increase at low temperatures, thus decreasing their switching losses. FIG. 6 shows the reduction in driver circuit current of the TC4422 as a function of frequency for a load capacity of 0 nF and 7.5 nF at temperatures of 300 K and 77 K.
The efficiency of semiconductor diode lasers can be high (20% to 60%). Nevertheless, they generate considerable heat. A dissipation energy of 45 Wh evaporates one liter of liquid nitrogen (LN2). But one can here make use of the xe2x80x9cload sheddingxe2x80x9d property of LN2, which can be generated in off-peak hours. For many applications where high laser power is not continuously required, the cost of liquid nitrogen may be less than that of additional laser diodes, which would be required for normal temperature (300 K) operation.